[PATCH] libbpf: fix BPF_MAP_TYPE_PERF_EVENT_ARRAY auto-pinning

* [PATCH] libbpf: fix BPF_MAP_TYPE_PERF_EVENT_ARRAY auto-pinning
@ 2022-02-22 20:42 Stijn Tintel
  2022-02-23  2:36 ` Song Liu
  0 siblings, 1 reply; 6+ messages in thread
From: Stijn Tintel @ 2022-02-22 20:42 UTC (permalink / raw)
  To: toke, netdev
  Cc: bpf, linux-kernel, kpsingh, john.fastabend, yhs, songliubraving,
	kafai, andrii, daniel, ast

When a BPF map of type BPF_MAP_TYPE_PERF_EVENT_ARRAY doesn't have the
max_entries parameter set, this parameter will be set to the number of
possible CPUs. Due to this, the map_is_reuse_compat function will return
false, causing the following error when trying to reuse the map:

libbpf: couldn't reuse pinned map at '/sys/fs/bpf/m_logging': parameter mismatch

Fix this by checking against the number of possible CPUs if the
max_entries parameter is not set in the map definition.

Fixes: 57a00f41644f ("libbpf: Add auto-pinning of maps when loading BPF objects")
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
---
 tools/lib/bpf/libbpf.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 7f10dd501a52..b076ab728f0e 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -4750,7 +4750,7 @@ static bool map_is_reuse_compat(const struct bpf_map *map, int map_fd)
 	struct bpf_map_info map_info = {};
 	char msg[STRERR_BUFSIZE];
 	__u32 map_info_len;
-	int err;
+	int def_max_entries, err;
 
 	map_info_len = sizeof(map_info);
 
@@ -4763,10 +4763,15 @@ static bool map_is_reuse_compat(const struct bpf_map *map, int map_fd)
 		return false;
 	}
 
+	if (map->def.type == BPF_MAP_TYPE_PERF_EVENT_ARRAY && !map->def.max_entries)
+		def_max_entries = libbpf_num_possible_cpus();
+	else
+		def_max_entries = map->def.max_entries;
+
 	return (map_info.type == map->def.type &&
 		map_info.key_size == map->def.key_size &&
 		map_info.value_size == map->def.value_size &&
-		map_info.max_entries == map->def.max_entries &&
+		map_info.max_entries == def_max_entries &&
 		map_info.map_flags == map->def.map_flags &&
 		map_info.map_extra == map->map_extra);
 }
-- 
2.34.1


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